A new study provides some of the first documented evidence that the risk of big, concentrated tornado outbreaks is on the rise. The lead author, Florida State tornado expert James Elsner, told me, “I think our results suggest a climate change signal on U.S. tornado activity.”

If these results hold up, it will be like an F5 twister hitting the contentious tornado-climate discussion. Or maybe it will be like multiple supertornadoes hitting it, kind of like the new movie, “Into The Storm,” which, coincidentally enough, opens Friday.

Dr. Kevin Trenberth, the former head of the Climate Analysis Section of the National Center for Atmospheric Research, emailed me that “the study is plausible.” Both Trenberth and another top climatologist, Dr. Michael Mann, agree that:

This latest study provides further evidence for what many climate scientists have already surmised: that climate change is substantially altering the atmospheric environment in which thunderstorms and tornadoes form, increasing risk of major tornado outbreaks when conditions are ripe.

Mann adds, “The findings, moreover, undermine the claim made by some contrarians that human-caused climate change will reduce tornado activity and risk.”

After April 2011 saw records set for most tornadoes in a month and in 24 hours — “The Katrina of tornado outbreaks“ — many people, including many climatologists, wondered if there was a connection to climate change.

Tom Karl, the director of the National Climatic Data Center, explained in a 2011 email that “what we can say with confidence is that heavy and extreme precipitation events often associated with thunderstorms and convection are increasing and have been linked to human-induced changes in atmospheric composition.”

Insured losses due to thunderstorms and tornadoes in the U.S. in 2013 dollars. Data and image from Property Claims Service, Munich Re.

Tornadoes “come from certain thunderstorms, usually super-cell thunderstorms,” Trenberth explained in an email last year, but you need “a wind shear environment that promotes rotation.” Global warming, it was thought, may decrease the wind shear and that may counterbalance the impact on tornado generation from the increase in thunderstorm intensity. But the Stanford study found that most of the decline in wind shear occurs on days that weren’t suitable for tornado formation anyway.

Many scientists would agree with the December assertion of Penn State meteorology professor Paul Markowski and National Severe Storms Laboratory senior research scientist Harold Brooks that, “Because of the inconsistency in [historical tornado] records, it is not known what effect global warming is having on tornado intensity.”

But Florida State University (FSU) researchers led by Elsner have worked hard to reanalyze the historical tornado data to make them more consistent. Elsner said last September, “The risk of violent tornadoes appears to be increasing.” In particular, the trail of destruction from tornadoes appears to getting longer and wider. The FSU news release noted, “The Oklahoma City tornado on May 31, 2013, was the largest tornado ever recorded, with a path of destruction measuring 2.6 miles in width.”

Elsner presented his findings at the annual meeting of the American Geophysical Union in December, which I discussed here. As LiveScience reported at the time:

Beginning in 2000, tornado intensity — as measured by a twister’s damage path — started rising sharply, said Elsner, of Florida State University. “I’m not saying this is climate change, but I do think there is a climate effect,” he said. “I do think you can connect the dots.”

Elsner’s new study, “The increasing efficiency of tornado days in the United States,” in the journal “Climate Dynamics,” adds more evidence. That study concludes:

The bottom line is that the risk of big tornado days featuring densely concentrated tornado outbreaks is on the rise. The results are broadly consistent with numerical modeling studies that project increases in convective energy within the tornado environment.

Here is a key figure from the study calculating “the percentage of all tornadoes during a given year that occur on big days,” where big days are defined as days with “at least four, eight, 16, and 32 tornadoes”:

Fig. 4 Annual proportion of all tornadoes each year that occurs on big days. Proportions are computed for days having at least 4, 8, 16, and 32 tornadoes.

And so we now have some evidence that the risk of big, concentrated tornado outbreaks is on the rise, with climate change as the plausible suspect. Elsner wrote me, “I think our results suggest a climate change signal on U.S. tornado activity. Yet it is possible that at least some of the trend is due to changes in reporting practices.” He points out that the article links to numerous recent studies: “These findings and speculations are broadly consistent with numerical modeling studies (Genio et al. 2007; Trapp et al. 2007; Diffenbaugh et al. 2013) of future tornado environments especially those indicating that when deep convection occurs it may more likely become severe (Klooster and Roebber 2009).” All citations can be found here.

Here are the rest of Dr. Trenberth’s comments on the study:

The study of tornadoes is fraught with difficulty owing to the tremendous changes over time of where people are and how they are affected (how much damage is done). If a tornado occurs in an open field and no-one sees it, did it really happen? So the database is inhomogeneous. The authors try to get around this by dealing with only one threshold of occurrence, but there is no doubt that the data are not complete.

We also know that tornadoes come from thunderstorms and they occur most often in spring east of the Rockies in association with warm moist unstable air from the Gulf of Mexico but with pronounced wind shear in the vertical. The latter is associated with the Rockies and their juxtaposition to the Gulf; a unique situation and the reason why more tornadoes occur in the U.S. than anywhere else. The clusters of violent thunderstorms (supercells) and associated tornadoes occur when the storm track is positioned just right relative to the Gulf, and this varies a lot from year to year. A persistent favorable storm track, as is more apt to occur with La Nina condition in the Pacific, can lead to many widespread tornado outbreaks, as occurred in 2011. These physical aspects of the problem are mostly absent from this study, which instead focuses on statistical analysis of the (flawed) data.

Because 2011 was such an exceptional year, it influences the results, and yet it may or may not be part of a trend. The natural variability is so large that confidence in results is limited.

That said, the study is plausible. And it does link the statistics in a general way with the changes expected with climate change, namely that more moist unstable warm air is available to fuel the thunderstorms if everything else comes together. So the study finds that when conditions are favorable, we apparently get more tornadoes now than we used to.

If this result stands up, it will be a bombshell. I have been following the literature, the data, and expert analysis about the climate/tornado link for a few years. My original conclusions still stands:

When discussing extreme weather and climate, tornadoes should not be conflated with the other extreme weather events for which the connection is considerably more straightforward and better documented, including deluges, droughts, and heat waves.

Just because the tornado-warming link is more tenuous doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.

In general I still think it’s best to avoid statements like “global warming is to blame for” or “global warming caused” or “this is evidence of global warming,” with regard to tornadoes. I like the formulation of Climate Central’s headline on Elsner’s study, “Tornado Outbreaks Could Have a Climate Change Assist.”

Finally, while tornadoes will continue to grab the headlines wherever they flatten cities and take lives, it is virtually certain that other extreme events — and ultimately the permanently changed climate — will cause the greatest harm attributable to human emissions of greeenhouse gases.

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